Public safety agencies around the world typically use relatively narrow band, LMR technologies such as P25 to communicate voice information and some data traffic. Characteristics of this technology are long range and high quality voice. Today, Public Safety is considering the use of new technologies, such as LTE, capable of high rate data enabling mobile data applications. This form of technology is relatively short range compared to relatively narrow band technologies. In a move to capture the benefit of both technologies, methods of integration are being considered.
Typically, LMR systems are deployed over wide areas where public safety operation is expected including both populated and unpopulated areas. Typically, cellular systems are deployed over populated areas or areas where revenue can be generated through user traffic. There are areas where LMR exists but no cellular. There are areas where cellular exists but no LMR. There are areas where both LMR and cellular exist. Historically there have been innovations that seek to selectively communicate calls over either cellular or LMR depending upon which bearer is available.
LMR systems typically exist in two forms. The first is referred to as conventional. In this configuration a radio channel is used by a group of terminal devices where the communication is conducted by a repeater that is typically located on a physically high site. The user of the terminal will typically make a manual choice of channel number through choosing that channel via a selection knob on the terminal. Upon pressing a push to talk button on that terminal, the voice is then received by any other terminal that is configured to listen to that channel. In its simplest form, any terminal listening to the channel will unmute. This is one way of creating group communications.
Still considering conventional communications, a group can also be created according to a group identity. The group identity is normally a specific number that identifies the group. This identity is sent along with the voice information and now any terminal that is both listening to the channel and a member of this group will unmute.
The other form of system typically used in LMR is trunked. In this case, there is typically one control channel and several traffic channels. In effect, a trunked system is a centrally controlled network or trunked controller. All terminals not in a voice call will listen to the control channel. If a call is initiated then they are sent to a traffic channel. Any trunked terminal wishing to access the network must first register with the network. This represents a form of security to ensure the terminal is permitted on the network. Registration normally takes place at switch on of the terminal or when the terminal enters coverage of the control channel. All trunked terminals must register.
Following registration, all trunked terminals must then identify which groups they are a part of. This is required so that if a call for that group is initiated then all terminals in that group can be sent to the correct traffic channel. This group information (referred to as group affiliation) is knowledge held by the trunked controller.
A similar problem exists in the case of conventional radio wherein the management of groups remains a key issue when seeking to design an integrated solution across LMR and IP. Key to operation is the management of groups. Here we describe the management of groups through the interpretation of signalling and establishment of optimal routes required to maintain call quality and seamless connectivity.
Ideally, solutions are required that integrate LMR RF channels and IP capable bearers in such a way that a user is unaware of the communication bearer being used. In other words, the terminal always appears to function as a normal LMR radio whether conventional or trunked.
Previous attempts to enable LMR across a cellular network generally selects between an LMR voice call or a normal cellular call. In other words, the protocols for each path are different and only one path is chosen at any time. In some cases LMR is described in the context of tunnelling LMR information through an IP pipe. A key problem exists relating to the management of control information, path set up and seamless connectivity. Interpretation of signalling and establishment of optimal routes are required to maintain call quality and seamless connectivity.
One approach that has been used in the past is a gateway between LMR and Cellular. Methods of this type are subject to a number of draw backs including; a mismatch of audio quality on either side, an incompatible feature set on either side and loss of ability to establish end to end encryption.
The first bearer is LMR and may be P25 (APCO 25), Tetra, DMR (Digital Mobile Radio) or generally any form of relatively narrow band protocol. The second bearer may be LTE (3GPP Long Term Evolution) or Wifi or generally any form of wireless bearer capable of relatively high data rates to communicate IP packets. In an example system P25 is used as the LMR protocol. Other bearers may be present and participate.
Typically in an LMR network a channel is assigned to a call and the mobile end point of that call is an LMR terminal with a unique ID. Typically in an IP network over cellular the channel is shared by many mobile end points such as smart devices which have IP addresses. By converging these technologies we shall associate the ID of an LMR terminal with an IP address.
An LMR system is also typically Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA). A channel can be defined as either, a frequency, frequency pair or time slot on a frequency or frequency pair.